I bought a pair of JBL SRX738 using the 18¨ 2268HPL (neodynium, differential drive). The measurements show a very low distortion even driven at high power (thanks to the differential drive); but a frequency response not so good at medium frequencies (#3dB rised level above 250Hz, with peaks).

The enclosure is obviously not built as an ¨audiophile¨ one, the panels are not so thick and not well damped...
I found the sound quiet colored and not very dynamic ... not punchy... my previous subwoofer (with two PR380T4 AUDAX in a ¨manifold¨ loading) was souding a great better!

For information I use a triamp system with a digital active crossover XTA.

So I would like to design a good enclosure for that 2268 - for a 40Hz to 350Hz range. Flat frequency response, low distortion and good ¨dynamic¨ response are what I am looking for.
I don't want a 20Hz sub.

After some simulations I was thinking about a 150 liters /40Hz bass-reflex enclosure.

What are your suggestions for loading that 2268, and what are your experiences with this driver?

Bjorno,
Thanks a lot! for all that simulations. The results seem nice.
If I understand well the project is a 200L /38Hz Bass Reflex? I am not so sure looking at the schematic diagram.
How can we explain the behavior above 500Hz?

For my project, I would keep the JBL CMCD-81H (horn loaded midrange 8¨ differential drive) for the 350Hz-2kHz range (this part is included in the SRX738).
So this 18¨ should be used in the 40-350Hz bandwidth (its waterfall and lack of distortion up to 2kHz are OK with it I believe, even if I agree with Rademakers that 18¨ are often used only as subwoofer).

Bjorno,
Thanks a lot! for all that simulations. The results seem nice.
If I understand well the project is a 200L /38Hz Bass Reflex? I am not so sure looking at the schematic diagram.
How can we explain the behavior above 500Hz?

For my project, I would keep the JBL CMCD-81H (horn loaded midrange 8¨ differential drive) for the 350Hz-2kHz range (this part is included in the SRX738).
So this 18¨ should be used in the 40-350Hz bandwidth (its waterfall and lack of distortion up to 2kHz are OK with it I believe, even if I agree with Rademakers that 18¨ are often used only as subwoofer).

Thanks

Hi Masillia,

Don't worry about the dips and peaks:

They are artifacts from the fact that the box when empty,was simulated without any damping materials.

To cure: Line the rear internal wall with a suitable layer of foam or a sheath of polyester and the ripple will in practice disappear...

As the simulation shows, you will reach 350 Hz without flaws but this depends on your box internal layout as you will reach the speaker driver mass-corner when at ~170 Hz where the FR starts sloping.

I would be very interested in looking at some measurements you performed on this SRX 728, and your opinion about the sound of that subwoofer.

In the SRX738 I found it not punchy, and colored in the upper bass - due I expect, to the enclosure. Using it in active mode.

Regards

Massilia,

The JBL TS parameters that you referenced are the ones that show the Xlim of 23 mm as the Xmax figure, bjorno used 8 mm as Xmax, which sounds right, though I’d like to see the source, as I have seen many discussions speculating as to what the actual Xmax is, usually guessing about 11 mm.

My actual experience with the SRX 728 was setting up a DBX DriveRackPA for a DJ using pink noise and Smaart. I did not get to listen to it with actual music, but the 728 and the 718 are well regarded for subs in their price range.

My test had filters engaged, so it does not show the upper range, the graph below from one of Wayne Parham’s sub shootouts shows the port artifacts better than my test, and the LF rolloff looks very much the same in both my test, Wayne's and the JBL published graph. Since the port artifacts are narrow dips, rather than peaks, they should not be very noticeable.

As you can see, the present tuning of the 738 looks to be just below 50 Hz, I would suggest lengthening the ducts to tune the Fb down around 38 Hz, which would reduce the 50 Hz “bump” that is “coloring” the sound.
A lower tuning will sound much more “damped”, and be easier to EQ.

Though tuning lower will reduce LF output in the 50 Hz range, the lower, more gradual extension may actually sound “louder”, worth a try before building a separate cabinet, which would disassociate the LF/MF (if you don’t cut the bass portion of the 738 off, ruining it’s resale value), not a good idea with such a high crossover point.

Regarding lack of “punch”, the time alignment difference at the crossover point between the woofer and mid horn could be the problem, the mid/high portion (or the woofer) may need delay added.

Using a test tone at the crossover point, reverse the polarity of the LF, add delay to the top portion in small increments, the point where the output is least on your dB meter is the correct delay, though it should not be more than about 1-3 milliseconds. A wavelength at 315 Hz is just over a meter, if you end up with a delay in the 7 ms range for the null, the delay is one full wavelength late. If adding delay makes the null less as delay is added to the top, the LF may need delay instead.

Put the polarity back to normal after the proper delay is determined.

If you have an RTA or dual FFT test gear, pink noise will show the reverse polarity null to be deepest at crossover point, pick an ISO center (315 or 400 Hz) if using an RTA The crossover point should be reset to the recommended point after the alignment is done.

The tests are best done outdoors, microphone on the ground about 2 meters from the cabinet front, midpoint between the mid horn and woofer, cabinet laying horizontal, both microphone and cabinet well away (at least four meters) from any walls.

Thanks a lot for all those explanations.
About the 23mm surely it's the maximum-before-damage excursion rather than max linear displacement.
When I did setup on the SRX738 I used a XTA digital crossover, and made measurements with 1/24e octave resolution. I used the delay according to JBL setup (for time-alignment between all 3 sections of the SRX738). It was measured outdoor.

Your method for the delay is very interesting, I will try it next time.